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d' calculation

I'm trying to calculate d' from an experiment that used a modified method of constant stimuli.  Essentially, it is a same-different paradigm in which S1 is presented in the first interval of a trial.  On half the trials the second interval is also S1 (same trials).  The remainder of the trials are equally distributed S1-S2, S1-S3, etc (in the current instantiation, we have five different values S2-S6), with S2 representing the smallest change and S6 representing the largest change (the change is a increment in the level of 2 harmonics in 5-harmonic complexes).  This seems like a clear example of what McMillan and Creelman refer to as a reminder task but it wasn't clear (at least to me) what the most appropriate way of calculating d' is from this type of paradigm.  Fitting a logistic function (Green, 1993) seems like a reasonable solution but I wonder if there were other possibilities. Thanks for any suggestions
Mitch Sommers
Mitchell S. Sommers, Ph.D.
Associate Professor
Dept. of Psychology
Washington University
Campus Box 1125
St. Louis, MO 63130
Phone: 314-935-6561
Fax: 314-935-7588
e-mail: msommers@artsci.wustl.edu